The houses
| City | Housing units | Built 1980+ | Median year built |
|---|---|---|---|
| Wentzville | 16,601 | 90.1% | 2008 |
| O'Fallon | 36,206 | 90.8% | 2000 |
| Lake St. Louis | 7,657 | 80.6% | 2000 |
| St. Peters | 25,557 | 74.7% | 1988 |
| St. Charles | 32,305 | 61.6% | 1986 |
| County total | 167,163 | 75.2% | 1994 |
U.S. Census Bureau, ACS 2020–2024 5-year, B25034/B25035.
What is in the openings: 1980s and early-90s houses often have aluminum-clad or primed wood double-hungs with the first generation of insulated glass, many with failed seals and some with rot at the sill where the cladding was never sealed; mid-90s to 2000s houses have builder-grade vinyl double-hungs with nailing fins, welded corners and spring balances — the ones that fog, stick, and crack at the welds on a west elevation. Both are insert candidates when the frame is straight; the 1980s wood with sill rot is a full-frame job. St. Charles city's old core — Frenchtown, the Main Street district, the streets near Lindenwood — is the exception: 6.8% pre-1940, brick and wood, and the city has its own historic-preservation review that we confirm before quoting a street-facing window there.
The County's permit rule
The St. Charles County Code (§500.150, adopting the 2021 International Residential Code with amendments) exempts from permit "Replacement of three (3) or less doors and/or windows where the opening size is not increased or decreased." Four or more, or any opening change, needs a permit. That applies in unincorporated areas — Defiance, the countryside around Augusta and New Melle, and much of the county west of Highway 40/61 outside the city limits. Inside O'Fallon, St. Peters, St. Charles, Wentzville, Lake St. Louis and Cottleville the municipality issues the permit and sets its own rule, which we confirm for your address. We pull the permit where one applies. See the permits guide.
Design conditions, from the County's own code table
The County's Table R301.2 sets the numbers a window and its install have to meet here: a ground snow load of 20 psf, an ultimate design wind speed of 107 mph, Seismic Design Category C, "Severe" weathering, a 30-inch frost line, a moderate-to-heavy termite zone, an air freezing index of 1,000 and a mean annual temperature of 55°F. The "Severe" weathering classification is the one that matters for a window: it is the reason sealant and flashing have to be rated for freeze-thaw, and it is why a cheaply installed 1990s window has pulled away from its fin at the corners.
Climate: colder, earlier, longer than the airport
The National Weather Service's freeze tables put St. Charles's median last spring freeze at April 10 and first fall freeze at October 18 — two weeks earlier in the fall than Lambert's November 1, because the airport sits in the urban heat island and the county does not. Chesterfield Spirit is similar (October 21). The county's heating season is a little longer and its summer just as humid (July dew point normal 66.8°F at Lambert). The ENERGY STAR North-Central spec — U-factor 0.25 or lower, SHGC 0.40 or lower, the zone EPA's map assigns the area — is right here, with the lower SHGC on the unshaded west elevations that new subdivisions have in abundance. See performance ratings.
The March 2024 hail
On March 14, 2024, supercells dropped 2.5-to-3-inch hail in O'Fallon and 3-to-3.75-inch hail reported in St. Charles County and Madison County — among the largest stones in the metro that day — along with roof and siding damage county-wide. A lot of St. Charles County windows were replaced on claims in 2024 and 2025; a lot of fogged-seal windows were also denied because the seal failure predated the storm. If you still have hail-marked frames and cracked glass from that event, the claim window on most policies is limited; storm damage window replacement covers documentation and the storm-versus-pre-existing line.
HOAs
Most of the county's post-1990 housing is in a subdivision with an architectural review covenant. It is not a permit, but it is often the stricter of the two for exterior color, grid pattern and whether a bay window may be added. Check the covenant before ordering; the quote can match the existing grids exactly if that is what the HOA wants.
What it costs here
| Job | Typical installed (2026) |
|---|---|
| Vinyl double-hung, insert, frame wall | $550 – $950 per opening |
| Fiberglass double-hung, insert | $850 – $1,350 |
| 1980s clad-wood with sill rot, full-frame vinyl or fiberglass | $800 – $1,650 |
| Picture or fixed unit, vinyl | $500 – $900 |
| Bay window (3-lite), vinyl | $2,800 – $5,500 |
| 1995 two-story, 14 openings, vinyl inserts | $7,000 – $12,100 |
| 1988 St. Peters two-story, 18 openings, fiberglass inserts | $13,600 – $21,600 |
No masonry premium, no historic premium, and lead-safe work only on the one house in four that predates 1978. See the cost guide or run the instant estimate.
Not ready to talk yet?
Start with the complete St. Louis replacement window guide, then comparing window quotes — St. Charles County is where the in-home sales appointments concentrate.
Frequently asked questions
Do I need a permit in O'Fallon or St. Peters? The County's three-window exemption applies only in unincorporated areas; each city sets its own rule. We confirm it for your address and pull the permit if one applies.
My 1990s vinyl windows are foggy but otherwise fine. Replace them? If the frames are straight and the welds intact, replacing the glass units is cheaper and gets the same clear view. If the frames have warped — common on west elevations — replace the window. See repair or replace.
Does lead-safe work apply to my 1996 house? No — the rule covers housing built before 1978.